A large number of locking devices exist for the prevention of breaking and entering into a dwelling or similar structure. Many of these locks are directed towards security devices for limiting the degree to which a door may be opened (as opposed to locks that prevent any opening whatsoever). That the ability to secure a door in a partially open position for observation purposes to determine whether a visitor is friend or foe is highly desirable is an understatement, indeed, security devices performing this function may be required by law, or may be necessary in order for a hotel or similar establishment to obtain a desirable rating classification.
Perhaps most familiar is the releasable chain arrangement, wherein a chain is secured at one end in a permanent fashion, usually to the door frame, while the other end is retained in a slotted member mounted on the door. Such chains have the well-established fault of being weak in construction, both in terms of the chain-link material, which may be easily cut with bolt-cutters or broken under force, and in terms of the strength of the chain mountings, which provide relatively small resistance to the door being kicked or pushed open and the chain being ripped from its moorings. Chain-type limiting devices have the further fault of permitting disablement by the extension of a hand or tool through the partially opened door and the consequent not so difficult unfastening of the releasable end of the chain from the door. In addition, chain locks may be difficult for the elderly and handicapped to operate, since they require a certain degree of dexterous manipulation and precision of alignment to engage the chain in the slotted member.
In an effort to overcome the limitations of the chain-type lock, there exist in the prior art a number of devices analogous to the chain lock for permitting a door to be securably opened only a limited distance. It is perhaps most common that such devices incorporate pivoting or hinged members that are in some fashion linked together. A simple example is shown in U.S. Pat. No. 3,940,956, issued to Stevens on Mar. 2, 1976. Steven's device consists of a first arm pivotally mounted to a door frame and a second arm pivotally mounted to the door. The mountings are so as to permit a horizontal pivoting. The free end of each arm has a hole. The holes of the two arms are aligned to receive the shackle of a common padlock, which links the two arms together thereby permitting only a partial opening of the door to the limit as reached by the two linked arms. This arrangement is awkward since it requires a supplemental lock for which a key or combination number must be used.
A more elaborate example is shown in U.S. Pat. No. 3,924,886, issued to Markovitch on Dec. 9, 1975. With this invention, a cylindrical "latchplate" having a T-shaped slot is mounted on the door frame. A "latchlink" T-shaped end is pivotally and slideably mounted on the door. When a secured partial door opening is desired, the T-shaped end of the latchlink is slid toward the door frame to be mateably inserted into the slot of the latchplate. The latchlink is then held within the latchplate as the door is opened and a simultaneous rotation and engagement of the latchlink within the latchplate occurs. This device is rather more complex than is necessary, and it is not clear that insertion and corresponding engagement of the latchlink within the latchplate would always be smooth and trouble free.
Numerous other door security devices are equally complex, if not more cumbersome in design and application, or have inherent in their design the possibility that the device may be rendered unsecure by an intruder's reaching through the partially opened door. (See for example U.S. Pat. No. 4,174,861, issued to Drake on Nov. 20, 1979, wherein a sliding pivot pin is employed having a threaded retaining head which it appears possible could be unscrewed by an intruding hand, thereby allowing withdrawal of the pin and a consequent deactivation of the security device.)
Because of the limitations associated with most presently available locks for permitting a partial, secured door opening, a substantial need still exists for such a door security device that is both simple in operation and construction, and that cannot be deactivated by an attempted intruder.